Builg is the name given to a hypothetical ancient people believed by some to have lived in south-eastern Ireland, around the modern city of Cork.

According to the historical scheme proposed by T.F. O'Rahilly the Builg are identical with or a sub-group of the Érainn or Iverni, who arrived in Ireland ca 500 BC and are attested in Ptolemy's 2nd century AD Geography. In O'Rahilly's view, they spoke a P-Celtic language known as Ivernic.

Their name appears to be related to both that of the Belgae of Gaul and Roman Britain, and that of the Fir Bolg of Irish mythology.

See also:

• Early history of Ireland